218 GEOLOGICAL SURVEY OF THE TERRITORIES. 



therefore, that it may have been a local formation. The high ridge 

 upon which station 30 is located, dividing the waters of the San Miguel 

 from those of the Animas, shows a volcanic cap throughout its entire 

 length. Crossing Bear Creek Pass, the red sandstones are again found 

 on the west side of the ridge, although their horizontal distribution is 

 limited. As on the east side, they are covered by trachyte, which shows 

 almost horizontal strata, slightly unconformable with the underlying 

 sedimentary beds. Here, as well as on Bear Creek, the sandstones 

 show a number of white interstrata, a characteristic that can be ob- 

 served throughout, in the same horizon, along the exposures on the 

 Animas. Quite considerable metamorphosis has taken place in the 

 upper strata, produced by the overlying trachytes. 



To the westward, opposite station 30, an interesting point may be ob- 

 served, where apparently the Carboniferous sandstones overlie the Creta- 

 ceous beds. This is one of the instances of unconformability alluded to. 

 The sandstones in question were deposited and raised to their present 

 position, erosion had taken place and carved out much of the present 

 shape, before Cretaceous waters invaded the region and the deposits of 

 Cretaceous Nos. 2 and 3 were formed. In the canon north of station 36, 

 the sandstones again crop out, overlaid by Cretaceous No. 1. Farther 

 down on the Rio Dolores, they form both the bed of the river and the 

 banks on either side lor some distance. In speaking of the volcanic area, 

 the isolated flows near stations 36 and 37 have been mentioned. The 

 fact that they do not staud in the same relation as regards time to the 

 preceding sedimentary deposits, receives an additional support, inasmuch 

 as the trachytes are found, to occur, covering the Lower Cretaceous, 

 which rests in turn upon the sandstones of the Upper Carboniferous. A 

 number of cases were observed where the volcanic material is intrusive, 

 which shall be referred to below. 



Near station 37, which is located on the red sandstone at an elevation 

 of 12,648 feet, this is the main rock, overlaid toward the north by the 

 Cretaceous beds, exteuding southward for a considerable distance. 

 About one-half mile north of station 37 the main anticlinal axis passes 

 from west to east, producing a considerable dip, from 10° to 18°, in 

 the strata. Either subsequent erosion, or a breaking of the strata, has 

 caused the small creeks of that locality frequently to find their courses 

 along the line of upheaval, whereas, in the quartzitic regions, the 

 steepest peaks owe their formation to it. A section takeu through 

 station 37, in a direction of about north 30° east (section III), will ex- 

 plain the stratigraphical conditions of the locality. The station itself 

 is located on a shaly stratum, 6, contained in the red sandstones, a, 

 From there the beds dip in a southerly direction, at an angle of about 

 12°. Traveling northward, however, a depression is soon reached, 

 marking the line of the anticlinal axis, and from that point the strata 

 dip off to the north. Numerous bluffs afford excellent exposures of the 

 beds, and the characteristic change of red and white strata is well cal- 

 culated to demonstrate the stratigraphy. Intrusive between the layers 

 of the red sandstone we find a bed of trachyte, /, apparently inter- 

 stratified, but pinching out, as the distance from the outcrop increases. 

 Two other deposits of trachyte were seen,/,/, the one cupping a narrow 

 ridge, the other forming a small plateau. Both of them cover Cretaceous 

 beds. Immediately upon the red sandstones the white Cretaceous, No. 

 1, follows, composed of a fine-grained, hard sandstone, c. Whether or 

 not an unconformability exists there, was impossible to decide, as the 

 debris from the latter sandstone obscured the junction at all points 

 visited, and dense timber prevented any extensive view. Above No. 1 



